This invention relates to a process for producing isophthalic acid by oxidation in aqueous medium of m-toluic acid or mixtures of m-toluic acid with m-xylene and/or other partially-oxidized derivatives thereof such as m-tolualdehyde.
Isophthalic acid is a valuable intermediate in the manufacture of polyester and alkyd resins. Used initially as a simple substitute for phthalic anhydride, it has progressively gained a self-established position based mainly upon the fact that it imparts to those resins better properties with respect to e.g. toughness and resistance to heat and chemical agents.
The widely used starting material for the manufacture of isophthalic acid is m-xylene. Generally, most processes suitable for the oxidation of p-xylene into terephthalic acid can be adapted to the transformation of m-xylene into isophthalic acid. Although such oxidizing agents as nitric acid or sulfur have been employed for this purpose, the most commonly used oxidant is molecular oxygen. In one process, m-xylene is reacted with air at high temperature and pressure in the presence of a lower alkanoic acid, e.g., acetic acid, as a solvent, a salt of at least one heavy metal, such as cobalt, manganese or cerium, as catalyst, and a bromine-containing compound as promoter.
Although this process is applied successfully at the commercial scale, it has some serious drawbacks. For instance, as a result of the combined action of the bromine promoter, the acidic solvent and the high temperature, the reaction mixture is highly corrosive. Consequently, expensive alloys or metals such as titanium must be employed as construction materials not only for the reactor itself but also for various parts of the downstream equipment, e.g., the heat exchangers to condense exit vapors and to some extent the centrifuge or any other solid-liquid separation device used to separate isophthalic acid. Another and still more serious drawback of this prior process arises from the fact that, under the severe oxidation conditions utilized, the acidic solvent undergoes substantial degradation which heavily impairs the economics of the process as the cost of petroleum-based chemicals increases.
In this bromine-promoted process, the reaction mixture must be maintained under "substantially anhydrous conditions", especially when the feed consists of "partially oxidized hydrocarbons such as toluic acid" (U.S. Pat. No. 3,064,044). This requires that the mother-liquor from the separation of the desired diacid be further fractionated to recover the alkanoic acid solvent in anhydrous form for recycle. The residue from this fractionation comprises the catalysts together with some by-products and untransformed intermediates. Generally, the residue is incinerated whereby the catalysts are converted to metal oxides which can only be reconverted into reusable species with difficulty.
Several patents, e.g., U.S. Pat. Nos. 3,626,001 and 3,974,214, describe a process for the manufacture of isophthalic acid under much milder conditions. In the process, m-xylene is reacted with molecular oxygen in the presence of an alkanoic acid solvent, e.g., acetic acid, a cobalt catalyst and an alkanal, e.g., acetaldehyde, as promoter; no bromine-containing compound is added and the temperature applied is moderate so that the drawbacks of the former process do not exist. However, the aldehyde is used in relatively large amounts, and it is transformed during the reaction into the corresponding alkanoic acid which must be recovered, purified and commercialized for the process to be economically feasible.
In this latter process also, water is detrimental to the reaction. Still more harmful is phthalic acid produced in the system from o-xylene, which is always present as an impurity in the m-xylene feed. As a consequence, water and phthalic acid must be maintained at low levels, i.e., less than 5 weight percent when the feed is a mixture of m-xylene and m-toluic acid (U.S. Pat. No. 3,626,001, claim 1). This requires that the mother liquors from the separation of isophthalic acid be further processed according to an elaborate procedure which, briefly, comprises the steps of: (1) flashing the solvent which is then submitted to fractional distillation to remove water, (2) adding water to the residue containing the catalysts and contaminants such as phthalic acid, (3) treating the resulting aqueous solution with sodium carbonate to precipitate cobalt carbonate, (4) separating the precipitate from the aqueous solution containing sodium phthalate, and (5) treating the precipitate with acetic acid to regenerate cobalt acetate which then can be recycled (U.S. Pat. Nos. 3,673,154 and 3,919,306).
It is apparent from the foregoing description of the prior art that many of the problems arising in the manufacture of isophthalic acid result from the generalized use of an alkanoic solvent. Actually, for carrying out the transformation of m-toluic acid, which is a relatively high-melting and high-boiling compound, into isophthalic acid, which is an even less volatile and less fusible material, the use of a solvent is an essential requirement for several practical reasons which can be summarized as follows:
(1) Without a solvent, removal of the heat of reaction is made difficult by the tendency of m-toluic acid and isophthalic acid to form crusts and deposits on any cool surface, thereby precluding the use of conventional heat exchangers for controlling temperature.
(2) Without a solvent, the conversion of m-toluic acid into isophthalic acid must be severely limited in order to keep the reaction mixture as a workable slurry.
(3) Without a solvent, handling the reaction mixture and especially separating isophthalic acid therefrom are quite difficult tasks.
(4) Without a solvent, the formation of heavy by-products by condensation and/or addition reactions is generally enhanced, thereby adversely affecting the color and purity of the resulting isophthalic acid.
The solvent used to overcome these difficulties must fulfill a number of conditions. It should dissolve substantial amounts of m-toluic acid at the working temperature but not of isophthalic acid (to allow recovery of the latter by simple solid-liquid separation). It must be inexpensive, inert to oxidation, and sufficiently volatile to allow the removal of the heat of reaction by solvent vaporization. It should be non-corrosive, non-toxic, and it should not interfere with the reaction system. As already noted, acetic acid which is generally used in prior processes is not really inert; it is relatively corrosive, and it is becoming more and more expensive. In these respects, water is a much more attractive solvent. But when water is present in sufficiently great amount to act as a diluent, it generally interferes with the action of metal catalysts, especially cobalt and manganese. U.S. Pat. No. 2,907,792 describes a process whereby various organic substances can be oxidized by reaction with oxygen in the presence of large amounts of water. For instance, m-toluic acid can be transformed in high yield into isophthalic acid. However, this result is made possible only by the use of hydrogen bromide or other bromine-containing agents at high temperature so that, here again, severe corrosion problems are encountered. Moreover, with this process, non-oxygenated substrates such as xylene are transformed only in poor yield into the corresponding carboxylic acids.
Recently, a process which obviates most of the disadvantages of the prior processes has been described by the applicant for the oxidation of p-toluic acid and mixtures of p-toluic acid with p-xylene and/or other partially-oxidized derivatives thereof such as p-tolualdehyde, (see co-pending application Ser. No. 068,648 filed Aug. 22, 1979). In this process, water is used instead of acetic acid as a solvent in such quantities as to maintain the reaction medium as a homogeneous liquid solution. The conditions are mild despite the fact that no use is made of any promoter. To ensure oxidation in such an aqueous system, the concentration of metal catalyst, i.e., manganese and/or cobalt, must be carefully adjusted so as to be above a well-defined minimum value which depends on the catalyst used and on the composition of the reaction mixture and below a given maximum value. However, if one attempts to apply this process for the oxidation of m-toluic acid, with or without the presence of m-xylene and/or other precursors of isophthalic acid, generally no reaction takes place even when the concentration of catalyst is in close accordance with the requirements of that process.